The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Generally, an automatic transmission achieving more speed stages has been developed to enhance fuel economy and optimize drivability. Recently, increase of oil price is triggering a hard competition in enhancing fuel consumption of a vehicle.
Therefore, many researches for reducing weight and enhancing fuel economy through downsizing of an engine and for securing drivability and fuel economy through multiple speed stages of automatic transmissions have been developed.
However, in the automatic transmission, as the number of speed stages increase, the number of internal components (particularly, planetary gear sets) increase, and as a result, a length of the transmission increases. Therefore, mountability, cost, weight, transmission efficiency, and the like may still deteriorate.
In this aspect, in recent years, 8-speed automatic transmissions tend to be implemented and the research and development of a planetary gear train capable of implementing more speed stages has also been actively conducted.
However, a conventional 8-speed automatic transmission typically includes three to four planetary gear sets and five to seven control elements (frictional elements). In this case, since the length of the automatic transmission increases, mountability may be deteriorated.
Accordingly, in order to increase the number of shift-stages of the automatic transmission, a double row structure in which the planetary gear set is disposed on the planetary gear set is recently adopted, or a dog clutch instead of a wet control element is recently applied, but in this case, we have discovered that there is a problem that an applicable structure is limited, and deterioration of shift sensitivity is involved due to the application of the dog clutch.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the present disclosure and therefore it may contain information that does not form the prior art that is already known to a person of ordinary skill in the art.